1. Field of the Invention
This invention relates to a radiation curable composition which is useful as a protective coating for optical fibers, i.e., a primary coating. This invention further relates to the use of an oligomer containing free hydroxyl functionality to prepare such radiation curable primary optical fiber coatings possessing excellent mechanical properties and a very low glass transition temperature. This invention further relates to a coating for an optical fiber which has satisfactory mechanical properties including low modulus, and high elongation. This invention further relates to a fiber with this primary coating preferably with excellent micro-bend resistance when compared to conventionally coated fibers.
2. Technology Review
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has significantly impacted local area networks (i.e., fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver even greater volumes of information in the form of data, audio and video signals to residential and commercial users.
This form of transmission is done by sending a beam of light through an optically clear fiber. Because interference with the light beam or its partial loss during the transmission must be at a minimum to make the use of optical fibers a successful communications technology, the optical fibers must be protected from any environment which will cause loss of signal or distortion of the signal. Coating the fibers is one such technique. The optical fibers are coated to protect the fiber surface from damage which can result from abrasion or water, to maintain the fiber strength, and to prevent transmission loss resulting from micro-bending. The term micro-bending refers to random bends in the coated glass fiber. These random bends generally have a short period (<1 mm) and a small amplitude (typically a few microns). Micro-bending may result from mechanical manipulation, e.g., lateral stresses arising when the fiber is wound on a drum or cable, or changes in temperature, e.g., thermal expansion or contraction differences between the glass fiber and the coating over a wide application temperature range. Coating materials which will provide cured films on the optical fiber that has all of these properties is difficult to achieve because improving one property often results in the decrease in another property.
Optical fibers typically contain a glass core, a glass cladding, and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating which is applied immediately after the fiber is formed serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. In order for the loss in transmission to be as low as possible, the primary coating should maintain its flexible properties over a broad temperature range. Especially important is the low temperature flexibility. The low temperature flexibility can be obtained if the coating has a low glass transition temperature, Tg. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.